A solid-state image pickup device for performing digital-processing on image data, e.g. a digital still camera that handles still picture, and a digital video camera that handles a dynamic picture, performs photoelectric-conversion on an optical image obtained through an image pickup optical system by a photoelectric conversion element, and performs digital-processing on the obtained image signal. As an area sensor where image pickup elements including the photoelectric conversion element and its accumulated charge retaining section are arranged in crosswise matrix, generally used are an MOS (Metal Oxide Semiconductor) image sensor and a CCD (Charge Coupled Device) image sensor. The effective incident light quantity of these area sensors is limited to a narrow area. Therefore, when an object with a large difference between light and shade is picked up by those area sensors, there are generated such a phenomenon in the bright part that the image is overexposed (referred to as whiteout hereinafter) and such a phenomenon in the dark part that black is crushed (referred to as black crushing hereinafter). This narrows the dynamic range of the image. In order to avoid such phenomena and widen the dynamic range of the image, conventionally, a long exposure signal and a short exposure signal are synthesized as depicted in Patent Literatures 1-3.
FIG. 16 is a characteristic chart that shows relations between the incident light quantities and the output signal levels of a long exposure signal Slong, a short exposure signal Sshort, and a composite signal Smix obtained by synthesizing those signals, respectively. A is an area with less incident light quantity, B is an area where saturation of the long exposure signal Slong starts, and C is an area where the long exposure signal Slong is completely saturated. In an image (object) with a significant difference between the light and shade, there are the areas A, B, and C mixed within the same image.
The long exposure signal Slong exerts an effect of improving the sensitivity of the area A (in the dark part with less incident light quantity), while generating a bad effect such as whiteout in the areas B and C (the bright part where the sensitivity is saturated). The short exposure signal Sshort exercises an effect of improving the sensitivity for the relatively bright part of the area object, while having a bad effect of generating the black crushing on the part with less light quantity. As described, the dynamic range is narrow when one of those signals is employed alone, and it is difficult to achieve clear image depiction of the image (object) with a significant difference between the light and shade (high contrast), which has the areas A, B, and C mixed in the same image.
Meanwhile, with the composite signal Smix obtained by adding the long exposure signal Slong and the short exposure signal Sshort, image information can be captured with less deflection over the area A to the area C, which allows a high contrast image to be depicted. That is, a wide dynamic range can be achieved.    Patent Literature 1: Japanese Unexamined Patent Publication H2-174470 (p. 2, FIG. 3)    Patent Literature 2: Japanese Unexamined Patent Publication H7-95481 (pp. 3-5, FIG. 2)    Patent Literature 3: Japanese Unexamined Patent Publication H11-75118 (p. 4-6, FIGS. 1-4)